This invention relates to containers, and particularly to plastics containers capable of withstanding substantial internal pressure.
Aerosol containers and other vessels under pressure have largely been made of metal, although recently plastics ones have been introduced. However, they have usually been of two-piece construction, the base being bonded to the rest of the body.
The favoured shape for such a container is cylindrical with an indented base, often known as a champagne base, since champagne bottles, which are under pressure when charged, have a cone shaped indent at the bottom.
While this shape can be applied to a plastics container, it has been found that even better resistance to internal pressure can be achieved by making the indent, in axial section of the container, generally the shape of the Greek letter Omega. In other words, there is an undercut around the indent, but this of course poses problems in moulding.
It is the aim of this invention to answer these problems.
According to one aspect of the present invention there is provided a method of making a container with an undercut re-entrant base, the material of the container being susceptible to deformation when heated, wherein the base portion of an embryo container, at an elevated temperature, is expanded against a multi-part mould having at least three segments surrounding a tapered core movable to wedge the segments from a closely packed array to a mutually spread array, the undercut re-entrant base being shaped by the segments when spread, and being released when the segments are closely packed.
The expansion will preferably be achieved at least primarily by blow moulding, although the embryo container could initially be expanded in the basewards direction towards said core and segments by mechanical means inserted through an opening at the end opposite the base. This opening would subsequently receive the blow pin.
In the usual form the container in both embryo and finished form will be substantially a body of revolution, the core being co-axial therewith, and the segments all being similar with radial sides abutting when close-packed.
Preferably the segments will shape the re-entrant base into a profile of n-shape.
At least part of the edges of the segments which are exposed to the material of the container may be chamfered or bevelled to create channels which form ribs projecting outwardly from the re-entrant base surface.